The U.S. Environmental Protection Agency has ordered Enbridge to dredge three areas where oil is still accumulating, almost three years later.

"The dredging will prevent submerged oil from migrating to downstream areas where it will be more difficult or impossible to recover," says the EPA.

Enbridge also has to maintain sediment traps throughout the river to capture oil outside the dredged areas.

EPA’s surveys show that submerged oil can be detected throughout the 2 mile long, 700-acre Morrow lake, clearly growing from 325 acres in 2012 and 189 acres in 2011.

When the spill occurred, the heavy bitumen gradually sank to the bottom. Enbridge wanted to simply let the oil degrade "naturally."

In 2011, Exxon’s pipeline ruptured in the Yellowstone River, contaminating 70 miles with 63,000 gallons of tar sands oil. Luckily, that cleanup was easier because the oil was lighter and didn’t sink to the bottom of the riverbed.